This invention relates generally to filters for the removal of particulate matter from gas streams, and more particularly to a filter suitable for regeneration by exposure to microwave energy. While this application discusses the present invention in the context of a diesel engine exhaust filter, the invention is easily adaptable to filter particulates from other types of gas streams.
Diesel engines emit a hazardous, sooty exhaust that can be rendered less hazardous by using diesel particulate filters. The filters trap soot particles emitted by an engine and thereby prevent the particles from entering the atmosphere. However, the soot trapped by such filters builds up over time. As soot builds up in the filter, the effectiveness of the filter decreases, an increased pressure drop occurs across the filter, and the engine experiences an increased exhaust gas back pressure which decreases engine performance. Therefore, a clogged filter must periodically be either replaced or regenerated. Depending upon the speed with which the filter fills with soot particles, replacement of clogged filters is inconvenient and expensive. Therefore, periodic regeneration of the filter (i.e., removal of the trapped soot) is the preferred method of maintaining a clean filter.
There are several techniques for regenerating diesel particulate filters. The methods typically involve igniting the trapped soot particles in the filter and thereby burning the soot out of the filter. One technique involves the periodic release of a burning gas into the filter, as disclosed in U.S. Pat. No. 4,912,920 to Hirabayashi. Another technique utilizes electrical heating elements in contact with the filtering elements. An electrically regenerable filter is illustrated in U.S. Pat. No. 5,258,164 to Bloom, et al. Yet a third technique utilizes microwave energy to heat the filter and cause the particulates trapped in the filter to ignite and bum, thereby regenerating the filter. An example of the latter technique is disclosed in U.S. Pat. No. 5,087,272 to Nixdorf, wherein the regenerable particulate filter structure uses a monolithic filter element fabricated from silicon carbide whiskers capable of converting microwave energy to thermal energy.
Filter assemblies regenerated by a burning gas additive or electrical resistance heating typically use metallic structures to support the filtering element or to provide resistance heating. A perforated metal tube, screen, coiled wire, or similar structure is often used to provide support to a filtering element. For example, U.S. Pat. No. 5,258,164 to Bloom, et al. discloses an electrically resistive expanded metal sleeve 21 positioned between an inner filter element 20 and an outer filtering element 22 which is used to heat and thereby regenerate the filtering elements 20, 22.
However, a particulate filter regenerated by heating the filter with microwave energy must not contain any metallic components, because metallic components interfere with the microwave energy. Metals are also poor microwave susceptors and thus do not tend to heat up when exposed to microwave energy. Therefore, filters using microwave energy for regeneration must be formed from microwave compatible materials. The materials may be microwave susceptors which heat when exposed to microwave energy. It is not required, however, that a material which is a microwave susceptor be used. A microwave transparent material which does not interact at all with microwave energy is also suitable. Inorganic materials, such as glass or ceramic, are often the materials of choice, as they can be designed to assume a wide variety of forms. For example, ceramic materials may range in form from large solid bodies which are machined (if necessary) into the shape of the finished product to small flexible fibers which may be formed into thread or yarn-like material.
When using ceramic materials, numerous filter configurations are possible. Known filter assemblies may simply be modified by substituting ceramic materials for components previously made of metal. For example, a perforated cylinder could be formed of a ceramic material and used in place of a similar metallic component to support a filtering element. Alternatively, a monolithic ceramic filter may be formed. One type of monolithic filter is a "wall flow" filter where the gases are filtered by passing from one passage way through a thin ceramic membrane into an adjacent passageway (typically in the form of a honeycomb). Yet another type of monolithic ceramic filter is referred to as a "foam" filter where the gas is filtered during passage through a body having some selected porosity, thereby capturing any particles larger than the "pores" of the foam. In each of the above filter configurations, ceramic components are used to provide a rigid supporting structure to the filter, thereby eliminating the need for any metallic components.
Using ceramic filters or ceramic filter components as a supporting structure of a filter has several disadvantages. Specifically, ceramic materials, especially when formed as structures such as monolithic filters, are typically rigid and susceptible to cracking from either mechanical or thermal shock. The brittle ceramic components are unable to respond to stresses due to their brittle nature in a manner similar to metal components without experiencing cracking, either at a microscopic or a macroscopic level. This characteristic can be problematic with ceramic components used in applications which experience a great deal of vibration, such as on vehicles and running engines. The continuous vibrations can cause microscopic cracks in the ceramic material which grow over time and eventually lead to complete failure, i.e. breaking, of the component. A similar result occurs when the ceramic material is exposed to rapid thermal expansion or contraction. For example, in a regenerable filter, a sharp thermal gradient is created across the filter by the burning of the soot within the filter. As the filter components expand and contract in response to the changing temperatures, cracking of the ceramic material often results. The cracking of the ceramic material is especially problematic in filter assemblies which utilize inflexible ceramic components for support structures or as monolithic filters. In such filters, failure of the ceramic component typically results in the failure of the entire filter assembly.
A need thus exists for a microwave regenerable filter assembly which is resistant to failure due to mechanical and thermal shock, and which is easy to manufacture at a low cost.